424 GEOLOGY, 



though the absence of fossils cannot be regarded as proof of such origin. 

 Limestones and dolomites. — Of the lime, magnesia, soda, and potash 

 leached out of the surface-rocks and carried to the ocean in solution, 

 the lime is largely extracted to form the shells, skeletons, teeth, armor, 

 and other hard parts of sea-animals and sea-plants. These limy 

 parts are at length left on the floor of the ocean and become more or 

 less disintegrated and help to form beds of lime-mud and lime-sand 

 which in time are cemented into limestone (Figs. 349, 350, 351, and 352). 

 A larger proportion' ^of tftsixiagnesia remains in solution in the sea- 

 water, but in ways not yet well understood, the magnesia sometimes 

 unites with the-dinie to form dolomite, a double carbonate of lime and 

 magnesia (Ca,M^J.G03. This, change is sometimes local, and sometimes 

 affects great series of beds, rnore commonly the ancient ones than the 

 modern. Sometimes the dolomization appears to have taken place 

 long after the origmal limestone was formed and probably some- 

 times after it was Hf ted out of the sea, while in other cases it seems to 

 have taken place while the sediment was accumulating, or at least before 

 the next overlying beds were laid down. The potash in solution is to 

 sbme large extent taken up by the land- and sea-plants or is retained in 

 the clays, and through them becomes again incorporated in the sedi- 

 ments. The soda largely remains in solution in the sea-water. 

 I Precipitates. — ^AVhen a portion of the ocean-water is isolated in a 

 region where evaporation from the surface of the water is greater than 

 the rainfall on it, and the inflow from the tributary basin, the lime, 

 ilaagnesia, soda, potash, and other dissolved substances (solutes) are 

 concentrated until the water becomes saturated. The solutes are then 

 precipitated in the order in which they reach the point of saturation. 

 This order, when taken in strict and full detail, gives a very complex 

 series, but the leading deposits are calcium carbonate {limestone), 

 calcium sulphate {gypsum), and sodium chloride {halite or rock salt) 

 (see p. 375). Isolated lakes in arid regions may give rise to similar 

 deposits. It has sometimes been thought that the ancient limestones 

 were produced largely by precipitation from concentrated sea-water. 

 While this is probably the case in some instances and to some degree, 

 it has not been demonstrated that the great hmestone formations were 

 made to any large extent in this way. The more accepted view is 

 that the limestones in the main were made from organic remains. The 

 lime in solution in the ocean is chiefly in the form of the sulphate. 



